A flip chip is generally a monolithic surface mount (SM) semiconductor device, such as an integrated circuit, having bead-like terminals formed on one of its surfaces. The terminals, typically in the form of solder bumps, serve to both secure the chip to a circuit board and electrically interconnect the flip chip circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder bumps is required. The solder bumps are typically located at the perimeter of the flip chip on electrically conductive pads that are electrically interconnected with the circuitry on the flip chip. The size of a typical flip chip is generally on the order of a few millimeters per side, resulting in the solder bumps being crowded along the perimeter of the flip chip.
Because of the narrow spacing between adjacent solder bumps and conductors, soldering a flip chip to its conductor pattern requires a significant degree of precision. Widely employed for this purpose are reflow soldering techniques, which typically entail precisely depositing a controlled quantity of solder on a flip chip using methods such as electrodeposition, and then heating the solder above its liquidus temperature to form the characteristic solder bumps on the surface of the chip. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to metallurgically adhere, and thereby electrically interconnect, each solder bump with its corresponding conductor, forming what will be referred to herein as a solder connection.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the height of the solder connections after soldering. As known in the art, controlling the height of solder connections after reflow is often necessary to prevent the surface tension of the molten solder from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate, termed the stand-off height, is desirable for allowing penetration of cleaning solutions for removing undesirable processing residues, promoting the penetration of mechanical bonding and encapsulation (underfill) materials between the chip and its substrate, and enabling stress relief of the solder connections during thermal cycles. Solder bump position and height are generally controlled by the amount of solder deposited on the flip chip to form the solder bump and by the use of solder stops that limit the surface area over which the solder bump is allowed to reflow. Solder stops are typically formed by a solder mask on laminate substrates and printed dielectric on ceramic substrates. For laminate circuit boards, the solder mask is applied over the conductor pattern and an opening is formed in the mask to expose a limited portion of each conductor, which then serves as a bond pad for the solder bumps.
While solder stops are widely used in the art, trends in the industry have complicated their ability to yield solder connections that provide an adequate flip chip stand-off height. As flip chips become more complex, the number of bumps that must be accommodated along the chip perimeter has increased. In turn, the conductors to which the bumps are registered and soldered have become more closely spaced and narrower, e.g., a pitch of about 0.010 inch (about 250 micrometers) or less and line widths of about 0.004 inch (about 100 micrometers), yielding a line spacing of about 0.006 inch (about 150 micrometers) or less. Fine solder bump and conductor pitches complicate the design and fabrication of solder stops, particularly on laminate substrates with the result that pitches of less than 0.010 inch have not been widely used. Solder connections having adequate stand-off height have also become more difficult to consistently produce, which increases the difficulty of removing residues from between the chip and substrate, underfilling the chip with bonding and encapsulation materials, and promoting stress relief in the solder connections during thermal cycling. This difficulty is particularly evident on laminate circuit boards, because the requirement for a solder mask as a solder stop requires that a portion of the mask remains beneath the chip, which reduces the stand-off height of the chip by the thickness of the mask. For example, on a fine pitch pattern of 0.010 inch, the height of each solder connection may be about 0.0036 inch (about 90 micrometers), but the stand-off height is only about 0.003 inch (about 75 micrometers) for a typical solder mask thickness of about 0.0006 inch (about 15 micrometers).
Accordingly, it would be desirable if a method were available that was able to increase the stand-off heights of flip chips and other surface mount devices, and particularly those devices requiring a fine pitch solder bump pattern.